Andover is a town in Essex County, Massachusetts, United States. It was settled in 1642 and incorporated in 1646. It is part of the Boston-Cambridge-Quincy, Massachusetts-New Hampshire metropolitan statistical area.

According to the United States Census Bureau, the town has a total area of 32.1 square miles (83.2 km2), of which 31.0 square miles (80.3 km2) is land and 1.1 square miles (2.9 km2) (3.49%) is water. Significant water areas include the Shawsheen River and Haggetts Pond, located in west Andover, which serves as the town's reservoir. Haggetts Pond was originally set apart from other waters, but since the late 1990s has had waters added from the Merrimack River, which constitutes half of the town's northern border, to supplement the growing needs of the town. Andover is also home to the Harold Parker State Forest, the Charles W. Ward Reservation, the Harold R. Rafton Reservation, the Deer Jump Reservation (along the banks of the Merrimack), as well as a very small portion of Lawrence's Den Rock Park. The town also has several golf courses. Andover's town center is located approximately four miles south of the center of Lawrence, and is 22 miles (35 km) north of Boston and 30 miles (48 km) southeast of Manchester, New Hampshire. Andover contains the westernmost point of Essex County, along the Merrimack River. It is bordered by Lawrence to the north, North Andover to the northeast, North Reading and Wilmington to the south, Tewksbury to the southwest. Andover also borders Dracut to the northeast; however, the boundary is within the Merrimack River, and as there are no crossings of the Merrimack within the boundaries of Dracut, one must go through Methuen or nearby Lowell to enter Dracut, as is the case with Dracut's border with Tewksbury.

As of the census of 2000, there were 31,247 people, 11,305 households, and 8,490 families residing in the town. The population density was 1,007.8 people per square mile (389.1/km2). There were 11,590 housing units at an average density of 144.3 persons/km2 (373.8 persons/sq mi).